In information processing systems for computers, etc., a magnetic disk drive is widely used as an information storage unit. If a magnetic disk drive is used, recording or reading of information can be carried out with a magnetic disk by scanning a magnetic head on the magnetic disk.
However, in recent years, as high density recording on magnetic disks has been advanced, various problems have been reported. For example, decomposition of the lubricant molecules in an operation for a long period of time has become a more serious problem as the environment at the recording portion has become severer.
As a lubricant for magnetic disks, a compound of the following formula (A) is commonly known as a perfluoro-polyether (PTFE) compound having the functional end groups converted to —CH2OH hydroxyl group (wherein each of h and i is an integer of at least 1) (e.g. Non-Patent Document 1):HO—CH2CF2O—(CF2CF2O)x1—(CF2O)Y1—CF2CH2OH  (A)wherein each of x1 and y1 which are independent of each other, is an integer.
It is known by the document that the surface having a coating film of compound (A) formed by the application of the compound (A) on the substrate surface, is excellent in the nature (self-replenishing property) such that when a part of the coating film becomes defective, the compound (A) around the defective portion will cover the defective portion to replenish the defective portion. Further, it is known that a —CH2OH group present at the molecular terminal of compound (A) is involved in such self-replenishing and adhesive property.
As a perfluoro-polyether compound such as compound (A) has become to be used under a high temperature condition, a compound having a lower vapor pressure has been desired. In an attempt to accomplish a low vapor pressure, it has been attempted to increase the molecular weight, but when it is attempted to increase the molecular weight of the above compound (A), there has been a problem that the viscosity increases remarkably, whereby its application tends to be difficult. Further, there has been a problem that the proportion of —CH2OH groups per unit volume becomes low by the increase of the molecular weight, whereby the self-replenishing property tends to deteriorate.
As a compound to solve such problems, a compound of the following formula (B) has also been proposed (wherein each of h and i is an integer of at least 1), but compound (B) also has a problem that the viscosity is high.HO—CH2CH(OH)CF2O—(CF2CF2O)x2—(CF2O)Y2—CF2CH(OH)CH2—OH   (B)wherein each of x2 and y2 which are independent of each other, is an integer.
Further, each of compound (A) and compound (B) requires a —OCF2O— unit in its molecular structure. This unit is a unit which causes the decomposition reaction of the compound, and thus, the compound having such a unit has had a problem that it undergoes deterioration in its use (e.g. Non-Patent Documents 2, 3 and 4).
As a compound having no —OCF2O— unit, a compound of the formula (A) wherein —CH2OH end groups are —COOH groups, has been reported. However, such a compound wherein the terminals are —COOH groups, has had a problem that when left under a high temperature condition, —COOH terminals undergo CO2 removal, whereby polar end groups will be lost, and the self-replenishing property will deteriorate. Further, the compound having —COOH groups has had a problem that the acidity of —COOH groups is large, thus causing corrosion.
On the other hand, if pollutants are deposited on the disk surface, there will be a problem such that during the operation of the magnetic disk drive, such pollutants will scatter around, thus causing a trouble such as corrosion of the magnetic head. Various causes are conceivable for such deposition of the pollutants. One of the causes is considered to be derived from a lubricant layer to be formed on the magnetic disk surface for lubricating purpose. Namely, if the lubricant layer is thick and non-uniform, the thick portion of the lubricant layer is likely to be in contact with the magnetic head, whereby the lubricant layer will be scattered.
Heretofore, the lubricant layer was formed, for example, by applying, on a disk surface, a composition having a fluororesin lubricant dissolved in a fluorinated solvent such as a hydrofluorocarbon (hereinafter referred to as HFC) such as 1,1,2-trichloro-1,2,2-trifluoroethane (hereinafter referred to as R113) or 1,1,1,2,3,4,4,5,5,5-decafluoropentane, or a perfluorocarbon (hereinafter referred to as PFC) such as tetrafluorohexane or hexadecafluoroheptane.
R113 was widely used as it is non-flammable and has low toxicity and a dissolving power, and it is also excellent in chemical stability, but it became subject to regulation as a compound highly likely to deplete the ozone layer.
Further, PFC and HFC are compounds which became reduction targets together with carbon dioxide (CO2), dinitrogen monoxide, methane and sulfur hexafluoride (SF6) in the Kyoto Protocol adopted in Third Conference of the Parties to the United Nations Framework Convention on Climate Change in 1997, and their use is expected to be restricted.
On the other hand, a method of using HFE7100 (tradename, manufactured by 3M) being a fluorinated ether, as a solvent for a lubricant to be applied on a magnetic disk surface, is disclosed (Patent Document 2). However, such a fluorinated ether is not necessarily one which shows adequate solubility depending upon the type of the lubricant and has further had a problem that the compositional ratio of isomers changes, whereby a constant drying property may not be obtained.
Patent Document 1: JP-A-6-44557 (page 2)
Patent Document 2: JP-A-2001-187796 (Example 2)
Non-Patent Document 1: “C. Tonelli et al, J. Fluorine Chem.”, 1999, Vol. 95, p. 51-70
Non-Patent Document 2: W. Fong et al, “IEEE Transactions on Magnetics”, March 1999, Vol. 35, No. 2, p. 911-912
Non-Patent Document 3: J. Scheirs, “Modern Fluoropolymers”, John Wiley & Sons Ltd., 1997, p. 466-468
Non-Patent Document 4: P. H. Kasai, “Macromolecules”, 1992, Vol. 25, p. 6791